The proposed work is in four parts: a) the preparation and examination by electron microscopy of transporting epithelia, b) the use of computer-assisted morphometric procedures to measure cell dimensions and subsequently to construct quantitative three-dimensional models of cell shape, c) the phenomenological description of relationships between epithelial transport function and the shape of epithelial cells, and d) physiological experimentation on isolated epithelia to measure directly functional parameters predicted by the structure-function correlation studies. To date, morphologic studies on five different segments of the adult rabbit nephron have been completed and structure-function correlations have been established for three of those segments. Studies are nearly complete on the other two adult nephron segments and on the proximal nephron of newborn rabbit, adult nephron segments under varying experimental conditions, rabbit gallbladder, and frogs nephron. A computer model of cell membrane porosity under steady-state and transient flow conditions also has been completed.